Today, biomarkers play a key role in early diagnosis, risk stratification, and therapeutic management of cardiac diseases such as acute myocardial infarction (AMI) and heart failure [10-13]. Established biomarkers such as the cardiac troponins and b-type natriuretic peptides were mainly discovered by candidate approach [13, 14]. By contrast, the recent development of high-throughput molecular technologies that allow with a reasonable effort the analysis of whole transcriptomes, proteomes, and metabolomes of individuals at risk, may lead to the discovery of novel biomarkers in an unbiased approach [15, 16].
MicroRNAs (miRNAs) are a new class of biomarkers. They represent a group of regulatory elements that enable cells to fine-tune complex gene expression cascades in a wide range of biological processes, such as proliferation, differentiation, apoptosis, stress-response, and oncogenesis [1-7]. In the cardiovascular system, miRNAs are not only important for heart and vascular development, but also play an essential role in cardiac pathophysiology, such as hypertrophy, arrhythmia, and ischemia [8, 9]. Particularly, miRNAs are important regulators of adaptive and maladaptive responses in cardiovascular diseases and hence are considered to be potential therapeutical targets.
Since recently it is known that miRNAs are not only present in tissues but also in human blood both as free circulating nucleic acids and in mononuclear cells. This may be due to the fact that miRNAs expressed in diverse tissues or cells may be able to be released into circulating blood. Although the mechanism why miRNAs are found in human blood is not fully understood yet, this finding makes miRNAs to biological markers for diagnostics for various types of diseases based on blood analysis including acute coronary syndrome or hypertrophic cardiomyopathy. For example, Ai et. al (Biochem. and Biophysical Research Communication, 2010, 391, 73-77) report that miRNAs are novel biomarkers for early diagnosis of acute myocardial infarction in plasma of humans. Expression levels of hsa-miR-1, hsa-miR-133a, hsa-miR-499 and hsa-miR-208a were found higher in subjects with acute myocardial infarction in comparison to healthy persons, patients with non-AMI coronary heart disease or patients with other cardiovascular diseases. Further, Wang et al. (European Heart Journal, 2010, 31, 659-666) found that the expression level of hsa-miR-1 was higher in plasma from human AMI patients compared with non-AMI subjects.
Thus, various miRNA markers have been proposed to indicate heart and cardiovascular system diseases. However, many of these markers have shortcomings such as reduced sensitivity, not sufficient specificity or do not allow timely diagnosis. Accordingly, there is still a need for novel and efficient miRNAs or sets of miRNAs as markers, effective methods and kits for the diagnosis of said diseases. Particularly, the potential role of miRNAs present in human blood as biomarkers for the diagnosis of heart and cardiovascular diseases has not been systematically evaluated yet.
The inventors of the present invention assessed for the first time the expression of miRNAs on a whole-genome level in patients with acute myocardial infarction (AMI). They identified novel miRNAs which are significantly dysregulated in blood of AMI patients in comparison to healthy controls. Said miRNAs predict AMI with high specificity and sensitivity. The inventors of the present invention also pursued a multiple biomarker strategy to circumvent the above-mentioned limitations by adding accuracy and predictive power. In detail, by using a machine learning algorithm, they identified unique miRNA signatures that predict diagnosis of AMI with even higher power, indicating that both, single miRNAs and especially complex miRNA signatures or sets derived from human blood can be used as novel biomarkers. The inventors of the present invention further found that some other single miRNAs directly correlate with heart infarct size estimated by Troponin T release.